Gas sensors utilizing an infra-red source and a corresponding infra-red detector are well known, in particular in the design of, for example, carbon dioxide and hydrocarbon gas detectors. Infra-red radiation emitted by the source is focused onto the detector, having passed through a chamber containing the gas under test, where some of the infra-red radiation will be absorbed by the gas. The absorption by a specific gas is a function of the wavelength of the infra-red radiation, and by careful selection of an appropriate optical band-pass filter at the detector, it is possible to determine the presence of a specific gas.
A particularly compact form of optical gas sensor has been described in GB 2372099 B to Dynament Limited, and is shown in FIGS. 1 to 3. A gas sensor 1 comprises an optical source 2 for emitting radiation in the optical spectrum and a detector 3 for detection of radiation emitted by the source 2. The source 2 and detector 3 are respectively located at opposite ends of an optical pathway 4 (FIG. 2) which pathway is defined by a circumferential chamber 5 and a central chamber 6 respectively defining a generally circumferential portion 4a of the optical pathway 4 and a generally radial portion 4b of the optical pathway.
As best seen in FIG. 3, the circumferential chamber 5 is defined by: a chamber base 7; an internal surface of an outer cylindrical wall 8 of the sensor housing; an external surface of an inner cylindrical wall 9 of the sensor housing; and a radial end wall 10. The central chamber 6 is defined by an internal surface of the housing base 11 and an internal surface of the inner cylindrical wall 9 of the sensor housing. The housing base 11 provides a planar reflective surface, in the central chamber 6.
Optical communication between the circumferential chamber 5 and the central chamber 6 is by way of a gap 12 in the inner cylindrical wall 9. To enhance reflection of radiation from the circumferential chamber 5 to the central chamber 6, a deflector element 13 provides a reflecting surface 14 which generally extends from the outer cylindrical wall 8 to the inner cylindrical wall 9.
The top 16 of the sensor housing includes a gas permeable window 17 to allow controlled diffusion of gas under test from the external ambient of the sensor housing to the optical pathway 4 in the chambers 5 and 6. The gas permeable window 17 typically comprises a disc shaped element of sintered flame arresting material that allows diffusion of gas but forms a combustion barrier so that the source 2 cannot accidentally act as an ignition source when the sensor is operating in a hazardous and combustible gaseous environment.
The detector 3 is mounted in the base 11 of the sensor housing and comprises a dual element pyroelectric detector. The detector elements 3a, 3b are arranged in a spaced relationship along a vertical axis V of the sensor housing, i.e. an axis parallel to the central axis defined by the inner and outer cylindrical walls 8, 9. This axial spacing of the detector elements 3a, 3b ensures that the characteristics of the optical pathway leading to each of the elements are substantially similar. Each element 3a, 3b includes a filter (not shown) to allow the transmission of optical radiation at selected frequencies or frequency ranges. This dual element configuration enables the sensor to operate with one reference or compensation detector to increase accuracy of the measurements, as will be described hereinafter.
In use, the incandescent source 2 emits infra-red radiation over a broad spectrum of frequencies. The reflective surfaces formed by the inner and outer cylindrical walls 8, 9 and the radial end wall 10 guide the infra-red radiation around the circumferential chamber 5. The non-focussing nature of the reflector surfaces means that positioning of the source 2 within the circumferential chamber 5 is not critical. Once the radiation reaches the other end of the circumferential chamber 5, via optical pathway 4a, radiation is reflected off the reflecting surface 14 of deflector 13 onto the radial inward optical path 4b, towards the detector elements 3a, 3b. 
A potential disadvantage with optical gas sensors, as opposed to other types of gas sensor, is that the detector output is not directly related to gas concentration. Therefore, complex signal processing must ordinarily be performed on the detector output in order for it to provide a reliable and accurate signal indicating the gas concentration.
In conventional gas detection equipment, this complex signal processing is generally carried out by electronics that is external to the sensor housing. The reason for this is typically an issue of space. It is desirable to adhere to industry standard dimensions in the construction of sensor housings to ensure backward compatibility with installed gas detection equipment.
Even though some limited signal processing may be carried out within the sensor housing, this is generally limited to relatively simple and straightforward functions such as zero adjustment, rectification and filtering to remove noise from the output. These functions do not require extensive signal processing capacity. Typically, these functions are carried out in the analogue domain. More complex processing such as the derivation of the gas concentration and linearization of the output signal taking into account temperature compensation, pressure compensation and other functions have hitherto been performed remote from the sensor housing.